1. Technical Field
The present disclosure relates to a surface light source having a configuration in which directionality of light emitted from light sources, such as a light emitting diode (hereinafter simply referred to as an “LED”), is expanded by a lens. The disclosure also relates to a liquid crystal display device in which the surface light source is disposed as a backlight at the back of a liquid crystal panel.
2. Description of the Related Art
In a backlight of a conventional large-size liquid crystal display device, many cold-cathode tubes are disposed immediately below the liquid crystal panel, and the cold-cathode tubes are used together with member(s) such as a diffuser plate and/or a reflector plate. Nowadays, the LED is used as the light source of the backlight. A luminous efficacy of the LED is improved, and expected as a low-power-consumption light source to replace a fluorescent lamp. In the light source for the liquid crystal display device, power consumption of the liquid crystal display device can be reduced by controlling lighting of the LED based on a video picture.
In the liquid crystal display device, many LEDs are disposed instead of the cold-cathode tube in the backlight in which the LED is used as the light source. Although the brightness can evenly be obtained on a surface of the backlight using the many LEDs, unfortunately cost increases because many LEDs are used. In order to solve this drawback, the approach that the number of LEDs is decreased by increasing an output per LED is promoted. For example, Japanese Patent Publication Laid-Open No. 2006-92983 proposes a light emitting device in which the surface light source having the even luminance is obtained by a small number of LEDs.
In order to obtain the surface light source in which the surface light source having the even luminance is obtained by a small number of LEDs, it is necessary to enlarge an illumination region that can be illuminated by one LED. In the light emitting device of Japanese Patent Publication Laid-Open No. 2006-92983, the light from the LED is radially expanded by the lens. Therefore, directionality of the light from the LED is expanded, and a wide range about an optical axis of the LED can be illuminated on the irradiated surface. Specifically, the lens used in the light emitting device of Japanese Patent Publication Laid-Open No. 2006-92983 is formed into a circular shape when viewed from above, and both a light incident surface and a light control output surface are rotationally symmetrical with respect to the optical axis. The light incident surface is formed into a concave surface. In the light control output surface, a portion near the optical axis is formed into a concave surface, and a portion outside the portion near the optical axis is formed into a convex surface.
Generally, in the LED, the largest amount of light is emitted toward a front surface direction of the LED chip. In the lens disclosed in Japanese Patent Publication Laid-Open No. 2006-92983, the light traveling from the light source toward the front surface direction is diffused by refraction on concave surfaces near the optical axis in the light incident surface and the light output surface. Therefore, a wide illuminance distribution is obtained while illuminance near the optical axis is suppressed on the irradiated surface.